Control systems for initiating throw on/off procedures for sheet-fed offset printing presses are generally known in the art. For example, references DE-AS 1 Q98 963, DD-PS 86 631 and DE 3 232 171 Al all disclose sheet-fed offset printing presses utilizing pressure-operated actuating means for throw-on and throw-off procedures. As disclosed in the above cited references, a blanket cylinder is placed in contact with a plate cylinder and/or an impression cylinder during each throw-on procedure, thus forming a nip at the point of contact, and the blanket cylinder is separated for the plate cylinder and/or the impression cylinder during the throw-off procedure.
The plate, blanket and impression cylinders all include grip edges for securing a sheet of paper, a printing plate, and an output for the ink supply, respectively. During both throw-on and throw-off procedures, the blanket cylinder is placed in contact with or removed from the plate and impression cylinders one at a time, i.e., in a sequential order. A grip edge zone exists on the surface area of a cylinder, defined by the circumferential area between the grip edges that is not covered by the paper sheet, printing plate or ink supply. In operation, the grip edge zones of opposing cylinders are in contact or immediately adjacent to each other (if the corresponding cylinders are not in contact) during throw-on and throw-off procedures, and, therefore, are considered to be in "registration." This registration of the grip edge zones is necessary in order to ensure proper printing of each sheet. When printing is interrupted, the blanket cylinder is thrown off the plate cylinder after the last sheet is printed. The blanket cylinder is thrown off the impression cylinder in order to ensure unneeded ink is not transferred to the blanket cylinder. Similar considerations apply to the throw-on procedure.
In conventional control systems, timers are used to activate the throw-on and throw-off procedures at a particular time and in a particular sequence. These timers are driven in synchronism with the press and include cams that control various changeover valves that regulate devices for actuating the throw on or throw-off of the blanket cylinder. Throw-on or throw-off is generally initiated when the grip zones enter the nip between two cylinders. In order to ensure that throw-on and throw-off procedures occur at the end of printing (i.e., a time when the relevant grip edge zones are in registration), triggering signals are adjusted to occur as the grip edge zones enter the nip and towards the end of printing.
A disadvantage of an actuating time for throw-off and throw-on that is in a fixed relationship to press position and located inside the grip edge zones is that in the case of a high speed press there is only a very short time slot available between the time of initiating the actuating device and the time at which the blanket cylinder has been completely thrown on or thrown off the impression cylinder or plate cylinder. Following conventional designs, the actuating device would have to be designed to operate at very high speeds, and thus tolerate very high driving pressure forces in order to ensure that throw-on and throw-off could be completed entirely within the time during which the grip edge zone of the relevant cylinders are in registration. Such a design would be very expensive, and therefore, is undesirable.